The invention concerns a process for marking semiconductor surfaces with a bar code produced by means of laser bombardment.
In such a process, known from the U.S. Pat. No. 4,585,931, a bar code whose individual bars have a depth of approximately 8 to 10 .mu.m is produced on the surface of a wafer by means of laser bombardment. The bar code, also known as barcode, is a combination of narrow bars, narrow gaps, wide bars and wide gaps.
In a process for marking semiconductor surfaces, known from the EP-A-O 134 469, alphabetic characters, numeric characters and other symbols are formed by means of circular melting points which are arranged at a distance relative to one another and are produced in the laser bombardment by means of melting on and partial evaporation of the semiconductor material. The circular melting points have depths of between 1 and 5 .mu.m and diameters between 70 and 75 .mu.m.
The mechanical reading of OCR characters (OCR=Optical Character Recognition) requires expensive devices in comparison with a bar code. Moreover, the ratio of defective decoding results to correct decoding results in a bar code is substantially smaller than in clear characters. For example, the typical substitution error rate in the known bar code "Code 39" is 1:3.times.10.sup.6, whereas, in clear, it is 1:10.sup.4. Finally, the space requirement for clear is relatively large with respect to character height and character density. The clear produced by means of laser according to the process known e.g. from the EP-A-O 134 469 can be produced in so-called softmark marking. Such softmark marking is understood to mean a melting on or melting around the surface of the semiconductor wafer at a small depth of approximately 1 .mu.m, in which material spatters are not produced and crystal defects do not occur in the immediate vicinity of the marking. The softmark marking of semiconductor wafers is particularly advantageous because this marking method can be applied at any desired point in time during the course of the chip production.
The laser marking of semiconductor surfaces with bar codes with the softmark technique could be controlled according to the prior art, but not its mechanical readability with commercially available bar code reading devices. The reason for this is that the reading signals in the detection of bar code symbols cannot be distinguished with the required reliability. The surface reflection and scattered light portion of wide and narrow bars are not reproducible in bar codes with the softmark technique. A reading reliability suited to the requirements of bar code, e.g. 0.003 per thousand in the known "Code 39", is not ensured in laser writing with the softmark technique.
In contrast, the hardmark marking of semiconductor surfaces with bar codes, known e.g. from the U.S. Pat. No. 4,585,931, can be decoded with optical sensing devices. In this case, the laser bombardment produces a depth marking with wide grooves for the individual bars. However, marking semiconductor surfaces by means of laser bombardment with the hardmark technique can only be applied by the wafer manufacturer because of possible shifting of the crystal and/or impurities in the surface of the semiconductor wafers. That is, the wafer producer still has the possibility of eliminating the defects by means of mechanical or chemical subsequent treatment after the application of the bar code. But this is not possible for the chip producer. Moreover, the necessary space requirement is relatively large with respect to character height and character density in marking by means of hardmark marking.